A lithium-ion battery that has large capacity and excellent cycle characteristics has been desired. Use of sulfur having a theoretical capacity of 1672 mAh/g for the cathode has been studied in order to achieve large capacity.
However, since sulfur has low electrical conductivity, it is necessary to provide conductivity in some way when using sulfur for the cathode.
For example, a method that mixes conductive particles (e.g., carbon) and a sulfur powder while heating the mixture at a temperature equal to or higher than the melting point of sulfur has been proposed (see Patent Document 1). In Patent Document 1, a cathode material is prepared by adding a binder resin to the resulting sulfur-carbon composite, and a nonaqueous solvent lithium-ion battery is produced using the cathode material.
However, the initial capacity of such a battery is about 1100 mAh per g of sulfur (i.e., only 66% of the theoretical capacity of sulfur is utilized).
A method that impregnates the pores of carbon with sulfur under vacuum with heating has been proposed (see Patent Document 2). In Patent Document 2, a cathode material is prepared by mixing the resulting sulfur-carbon composite with thio-LISICON (Li3.25Ge0.25P0.75S4) (i.e., sulfide-based solid electrolyte), and the performance of an all-solid-state lithium battery that utilizes thio-LISICON (Li3.25Ge0.25P0.75S4) as an electrolyte is evaluated.
However, the initial capacity (600 mAh/g) achieved when using acetylene black as carbon is smaller than that of the nonaqueous solvent lithium-ion battery disclosed in Patent Document 1. The theoretical capacity of sulfur is realized by 100% in the initial cycle when using mesoporous carbon referred to as CMK-3. However, since the irreversible capacity is as large as 46 to 61%, a sufficient capacity is not obtained in the second and subsequent cycles.
An electrochemical reaction in the cathode layer occurs only in a site where sulfur, carbon, and the electrolyte are present in intimate proximity to one another. A nonaqueous solvent lithium-ion battery is configured so that the electrolyte solution penetrates into the entire cathode layer to increase the electrochemical reaction area.
In contrast, since sulfur and carbon come in point-contact with each other when using a solid electrolyte, it is difficult to increase the electrochemical reaction area in an all-solid-state battery. The system disclosed in Patent Document 2 that uses acetylene black achieves low performance for the above reason.
The system that uses mesoporous carbon (CMK-3) has the following problem. Since CMK-3 has a large pore volume, CMK-3 can be impregnated with a large amount of sulfur, and conductivity can be efficiently provided to sulfur. It is considered that this compensates for a small reaction area to achieve a large initial capacity. However, CMK-3 has a drawback in that the irreversible capacity increases. Specifically, a sulfur active material repeatedly undergoes expansion and contraction during charging/discharging. If the solid electrolyte cannot follow the movement of the sulfur active material, contact between the solid electrolyte and sulfur or carbon is lost.
It is difficult for an all-solid-state battery to surpass a nonaqueous solvent battery in performance due to a small electrochemical reaction area and a poor capability to follow expansion and contraction of the active material. This mainly hinders practical utilization of an all-solid-state battery including a battery that uses a sulfur cathode.